Minor cleanup.

These are just minor cleanups I wanted to apply.

- Apply `gofmt` on `html.go`.
- Apply `goimports`-compatible formatting on `block.go` (space between standard and third party import groups).
- Move Travis build status image in a more pleasing, common location.
- Remove "Markdown pretty-printer output engine" from TODO steps; this is already done in [markdownfmt](https://github.com/russross/blackfriday#other-renderers).
- Remove unneeded trailing whitespace in README.